the number of coins in bounty need to be fixed prior to start of the bouty let say 100 million.
stakes also need to be set prior to start of bounty or they have no limit.
The stakes will depend on the number of coins allocated for the bounty and the number of the participants. If it is only a single campaign like a signature campaign, it will be easier to count. So the formula --> number of coins allocated for the bounty / number of the participants = stake for each participant.